1. Field of the Invention
The present invention relates to a liquid crystal driving circuit for driving a matrix type liquid crystal display panel and a liquid crystal display device.
2. Description of the Related Art
A liquid crystal driving circuit which comprises a semiconductor integrated circuit and serves to apply a video signal to a liquid crystal display device is manufactured by using a voltage-withstanding diffusion process of 10V or more in withstanding voltage. This is because when a liquid crystal panel is driven, positive and negative voltages must be alternately applied to the common electrode of the liquid crystal, that is, an alternating current driving operation must be performed on the liquid crystal panel in order to prevent deterioration of the liquid crystal.
FIG. 10 shows a conventional liquid crystal driving circuit disclosed in Japanese Laid-open Patent application No. Sho-63-304229. Referring to FIG. 10, the liquid crystal driving circuit is constructed by a semiconductor integrated circuit, and comprises a shift register circuit group 21 input a crystal clock signal XCL and a start clock pulse signal XSP, a data register circuit group 22 for latching video data PD1 to PD4 of n bits in parallel, a data latch circuit group 23 for latching the data of the data register circuit group 22 in accordance with a latch signal LCL, a decoder 24 for selecting gradation voltages of 2.sup.n values which are input from the external on the basis of the video data of n bits, a level shift circuit group 25, and analog switches 26 of 2.sup.n. Each output terminal selects one value from the gradation voltages of 2.sup.n values by the analog switch to apply a prescribed voltage to the liquid crystal panel. In order to perform an alternating current driving operation, the gradation voltage input from the external is varied every line or every frame.
The liquid crystal driving circuit alternately applies positive and negative voltages to the common electrode of the liquid crystal panel as described above, and thus a withstanding voltage which is two times or more as high as the threshold voltage of a liquid crystal driving thin film transistor TFT of the liquid crystal panel is required. Specifically, the threshold voltage of the liquid crystal TFT is usually equal to about 4 to 5 V, and thus in order to perform the alternating current driving operation, the liquid crystal driving circuit is manufactured by using the diffusion process having a high withstanding voltage of 10 V or more.
However, the conventional liquid crystal driving circuit has the following problems containing the above case.
As a first problem, when it is constructed by a semiconductor integrated circuit, the chip size is necessarily large. This is because the number of analog switches increases as the gradation number increases. For example, in the case of 8 bits of digital image data, 256 analog switches are required to each output. Further, since the load of a liquid crystal data line is increased (above 100 pF) and a liquid crystal writing time must be shortened (in the case of VGA of 640.times.480 pixels, the horizontal period is equal to about 30 eEsec, however, in the case of XGA of 1028.times.768 pixels is reduced to about 16 eEsec), the on-resistance of the switch is required to be lowered, and thus the transistor size must be large.
As a second problem, the power consumption is high. This is because n level shift circuits must be provided for each output and they need large current consumption. Usually, the level shift circuit has a disadvantage that the operation speed thereof is lower than other logic circuits and a transit current is very large. For example, in the case of 384 output terminal numbers and 256 gradations (8 bits), a transit current of 1mA flows in one level shift circuit, and thus a transit current of 384.times.8.times.1 mA=3.72A flows at maximum. Therefore, if the wiring resistance is high, the voltage drop would be large and some trouble may occur in the operation.